Voice communication systems, especially those with omnidirectional microphones, tend to pick up background noise. This background noise may include, for example, environmental noise such as other voices or sounds, system noise such as radiofrequency noise, and acoustic echo. The background noise is especially noticeable during periods when the user is not speaking.
To address this problem, conventional voice communication systems typically incorporate some form of voice activated switch or signal expander. Switching solutions use a switch to activate the amplifier only when a voice signal is present. However, sharp switching produces an uncomfortable “swooshing” sound heard by users. Expansion techniques produce a gradual change in gain, yielding a smooth transition between noise (no speech) and speech states.
Prior art expander systems are described for example in U.S. Pat. No. 6,549,630 entitled “Signal Expander with Discrimination Between Close and Distant Acoustic Source” assigned to Plantronics, Inc. FIG. 1 illustrates a functional block diagram of a conventional bi-stable signal expander system 100 comprising a microphone 105, an expander control stage 110, and a variable gain block (stage) 115. The microphone 105 is coupled to the variable gain block 115, and the expander control stage 110 is coupled to the microphone 105 and the variable gain block 115. The expander control stage 110 includes a detector 120 coupled between the microphone 105 and a first input 125 of a comparator 130. The comparator 130 has a second input 135 coupled to a reference (threshold) voltage source 140 for generating a reference voltage level Vref.
When an acoustic source 145 becomes active, the emitted sounds from acoustic source 145 will cause changes in the air pressure. The microphone 105 detects the air pressure changes and translates the air pressure changes into corresponding voltage changes (i.e., microphone output signals 150) that are detected by the detector 120. The detector 120 outputs the microphone output signal 150 as a detector output signal 155. The comparator 130 compares the voltage level of the detector output signal 155 with the reference voltage level Vref from reference voltage source 140. If the voltage level of the detector output signal 155 is below the reference voltage level Vref, then the comparator 130 generates an output signal 160 with a logical state that does not activate the variable gain block 115. As a result, the variable gain block 115 does not add gain to the microphone output signal 150. When the acoustic source 145 activates, the voltage level of the microphone output signal 150 rises.
The detector 120 detects the higher-level microphone output signal 150 and will, as a result, output a higher-level detector output signal 155. If the voltage level of detector output signal 155 rises above the reference voltage level Vref, the comparator 130 outputs an output signal 160 with a logic state that causes the variable gain block 115 to add gain to the microphone output signal 150. The variable gain block 115 then outputs the amplified microphone output signal as an audio output signal 165. Background noise is typically reduced by voice expansion techniques by approximately 10-12 dB during periods in which the user is not speaking.
The above-mentioned bi-stable signal expander systems have a fast attack and slow decay characteristic that causes the switches for controlling gain to respond quickly to a detected sound of a sufficient voltage level and to maintain the gain for a pre-defined time length (e.g., 150 ms to 200 ms) after the voltage level of the detected sound falls below the comparator 130 threshold. By maintaining the gain for the additional pre-defined time length, the quieter-sounding, trailing ends of the speech envelope are not cut off by the bi-stable signal expander system. These trailing ends are typically below the comparator threshold. However, noise is often amplified during the additional pre-defined time length when gain is maintained.
FIG. 2 illustrates a conventional variable gain signal expander system 200 including an expander control stage 205 coupled between the microphone 105 and the variable gain block 115. The expander control stage 205 includes a detector 210 coupled to an amplifier 215. The microphone output signal 150 is detected by the detector 210 and amplified by the amplifier 215. As a result, the amplifier 215 generates a control signal 220 with a magnitude that depends on the initial magnitude of the microphone output signal 150. The amount of gain provided by the variable gain block 115 to the microphone output signal 150 depends on the magnitude of the control signal 220.
However, these and other prior art expander systems have certain performance shortcomings. For example, there is high distortion in the transition mode between a signal with no speech and a signal with user speech (also referred to herein as a “voice signal”). Furthermore, attack and release times not independent or adjustable over a wide range. Thus there is a need for improved expander systems.